Add `OneElementFastSequence` to be used in `ConnectionPool` #1033

Job	Run time
gh-codeql	7m 27s
linux-integration-and-dependencies (postgres:16)	9m 38s
linux-unit (swift:5.7-jammy)	2m 51s
api-breakage	5m 20s
linux-unit (swift:5.8-jammy)	3m 21s
linux-integration-and-dependencies (postgres:14)	7m 49s
linux-unit (swift:5.9-jammy)	3m 6s
linux-integration-and-dependencies (postgres:12)	7m 50s
macos-all (postgresql@15, scram-sha-256, ~14.3)	2m 53s
linux-unit (swiftlang/swift:nightly-5.10-jammy)	3m 49s
macos-all (postgresql@15, scram-sha-256, ~15.0)	3m 39s
linux-unit (swiftlang/swift:nightly-main-jammy)	5m 12s
	1h 2m 55s

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